linux-kernel - Re: [PATCH 1/1] powerpc/iommu: Enable remaining IOMMU Pagesizes present in LoPAR

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Date:   Tue, 23 Mar 2021 18:41:09 +1100
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Leonardo Bras <leobras.c@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        Joel Stanley <joel@....id.au>, brking@...ux.vnet.ibm.com
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] powerpc/iommu: Enable remaining IOMMU Pagesizes
 present in LoPAR



On 23/03/2021 06:09, Leonardo Bras wrote:
> According to LoPAR, ibm,query-pe-dma-window output named "IO Page Sizes"
> will let the OS know all possible pagesizes that can be used for creating a
> new DDW.
> 
> Currently Linux will only try using 3 of the 8 available options:
> 4K, 64K and 16M. According to LoPAR, Hypervisor may also offer 32M, 64M,
> 128M, 256M and 16G.
> 
> Enabling bigger pages would be interesting for direct mapping systems
> with a lot of RAM, while using less TCE entries.
> > Signed-off-by: Leonardo Bras <leobras.c@...il.com>
> ---
>   arch/powerpc/include/asm/iommu.h       |  8 ++++++++
>   arch/powerpc/platforms/pseries/iommu.c | 28 +++++++++++++++++++-------
>   2 files changed, 29 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/iommu.h b/arch/powerpc/include/asm/iommu.h
> index deef7c94d7b6..c170048b7a1b 100644
> --- a/arch/powerpc/include/asm/iommu.h
> +++ b/arch/powerpc/include/asm/iommu.h
> @@ -19,6 +19,14 @@
>   #include <asm/pci-bridge.h>
>   #include <asm/asm-const.h>
>   
> +#define IOMMU_PAGE_SHIFT_16G	34
> +#define IOMMU_PAGE_SHIFT_256M	28
> +#define IOMMU_PAGE_SHIFT_128M	27
> +#define IOMMU_PAGE_SHIFT_64M	26
> +#define IOMMU_PAGE_SHIFT_32M	25
> +#define IOMMU_PAGE_SHIFT_16M	24
> +#define IOMMU_PAGE_SHIFT_64K	16


These are not very descriptive, these are just normal shifts, could be 
as simple as __builtin_ctz(SZ_4K) (gcc will optimize this) and so on.

OTOH the PAPR page sizes need macros as they are the ones which are 
weird and screaming for macros.

I'd steal/rework spapr_page_mask_to_query_mask() from QEMU. Thanks,




> +
>   #define IOMMU_PAGE_SHIFT_4K      12
>   #define IOMMU_PAGE_SIZE_4K       (ASM_CONST(1) << IOMMU_PAGE_SHIFT_4K)
>   #define IOMMU_PAGE_MASK_4K       (~((1 << IOMMU_PAGE_SHIFT_4K) - 1))
> diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> index 9fc5217f0c8e..02958e80aa91 100644
> --- a/arch/powerpc/platforms/pseries/iommu.c
> +++ b/arch/powerpc/platforms/pseries/iommu.c
> @@ -1099,6 +1099,24 @@ static void reset_dma_window(struct pci_dev *dev, struct device_node *par_dn)
>   			 ret);
>   }
>   
> +/* Returns page shift based on "IO Page Sizes" output at ibm,query-pe-dma-window. SeeL LoPAR */
> +static int iommu_get_page_shift(u32 query_page_size)
> +{
> +	const int shift[] = {IOMMU_PAGE_SHIFT_4K,   IOMMU_PAGE_SHIFT_64K,  IOMMU_PAGE_SHIFT_16M,
> +			     IOMMU_PAGE_SHIFT_32M,  IOMMU_PAGE_SHIFT_64M,  IOMMU_PAGE_SHIFT_128M,
> +			     IOMMU_PAGE_SHIFT_256M, IOMMU_PAGE_SHIFT_16G};
> +	int i = ARRAY_SIZE(shift) - 1;
> +
> +	/* Looks for the largest page size supported */
> +	for (; i >= 0; i--) {
> +		if (query_page_size & (1 << i))
> +			return shift[i];
> +	}
> +
> +	/* No valid page size found. */
> +	return 0;
> +}
> +
>   /*
>    * If the PE supports dynamic dma windows, and there is space for a table
>    * that can map all pages in a linear offset, then setup such a table,
> @@ -1206,13 +1224,9 @@ static u64 enable_ddw(struct pci_dev *dev, struct device_node *pdn)
>   			goto out_failed;
>   		}
>   	}
> -	if (query.page_size & 4) {
> -		page_shift = 24; /* 16MB */
> -	} else if (query.page_size & 2) {
> -		page_shift = 16; /* 64kB */
> -	} else if (query.page_size & 1) {
> -		page_shift = 12; /* 4kB */
> -	} else {
> +
> +	page_shift = iommu_get_page_shift(query.page_size);
> +	if (!page_shift) {
>   		dev_dbg(&dev->dev, "no supported direct page size in mask %x",
>   			  query.page_size);
>   		goto out_failed;
> 

-- 
Alexey